Most algae have dominant gametophyte generations, but in some species the gametophytes and sporophytes are morphologically similar (isomorphic). An independent sporophyte is the dominant form in all clubmosses, horsetails, ferns, gymnosperms, and angiosperms that have survived to the present day. Early land plants had sporophytes that produced identical spores (isosporous or homosporous) but the ancestors of the gymnosperms evolved complex heterosporous life cycles in which the spores producing male and female gametophytes were of different sizes, the female megaspores tending to be larger, and fewer in number, than the male microspores.
A single gametophyte moss plant can produce both sperm and eggs. This can occur on different parts of the same plant, one part producing sperm and another part producing eggs. However, a plant usually produces either all sperm-producing organs or all egg-producing organs at any one time. This way it doesn't breed with itself, promoting genetic variation. The female structure for producing eggs is known as the archegonium, and the male structure for producing sperm is known as the antheridium. Antheridia are tiny, typically stalked, club-shaped or spherical structures. Archegonia are bottle-like containers, their wall just one cell thick. Archegonia are typically formed in groups. Archegonia and antheridia are usually bundled in leaf rosettes similar to flowers, called perichaetia. Elongated club-shaped cell filaments called Paraphyse are sometimes found on the gametophyte, storing water and protecting the archegonia sand antheridia from drying up.
When the antheridia are ripe and the flower gets wet from rain, numerous antherozoids (spermatozoids / sperm cells), are released. Antherozoids are only able to move underwater. They swim using two threadlike tails. Some successfully end up on female gametophyte moss plants and are chemically attracted to the archegonium. Each archegonium holds one egg, in a swollen section called the venter. The sperm enter the archegonium through the narrow channel in its neck. Fertilization occurs in the archegonium to form a diploid zygote. Once one archegonium in a group has been fertilized, in many cases the others lose the ability to be fertilized. This is caused by an inhibitory hormone released from the fertilized archegonium.
After fertilization, the archegonium on the gametophyte plant becomes modified into a protective sheath around the young sporophyte. The sporophyte begins to grow by mitosis (diploid cell division) out of the top of the archegonium. It elongates and after a few cell divisions begins differentiation. At this point the sporophyte is practically a parasite on the gametophyte plant, although it may produce some food of its own via photosynthesis in the early stages of growth.
In wet conditions the spores can't travel very far. A tiny tooth-like structure around the mouth of the capsule controls the release of the spores. These structures, called the peristome, consist of one or two rows of teeth. They prevent the release of the spores during wet conditions by remaining closed. In dry conditions they open, releasing the spores.
The asexual sporophyte generation (2N) produces motilezoospores (N) which develop into male and female gametophytes.Whereas the sexual gametophyte generation (N) produces male andfemale gametes (N). The male gametophyte plant produces malegametes called spermatozoids or antherozoids. The female gametophyteplant produces female gametes (eggs). At fertilizationmale and female gametes fuse to form the zygote (2N) whichsubsequently develops into a young sporeling at the beginning thesporophyte generation.
After a number of cell divisions the microscopic malegametophyte plant develops several spermatangia (also calledantheridia), each spermatangium producing a single motilebiflagellate spermatozoid which is released into the seawater(Fig. 1.5: 6b, 8).
1: zoospore 2: embryospore 3: germination of embryospore4: newly formed gametophyte 5a: female gametophyte 6a: malegametophyte 6a: mature oogonium 6b: spermatozoid beingdischarged from antheridium 7: discharged egg attached tooogonium 8: motile biflagellate spermatozoid 9: fertilization10: zygote 11: 7-celled seeding or sporeling (sporophyte) 12:young seedling or sporeling 13: young sporophyte 14: robustsporophyte 15: mature sporophyte with sporangial sori. C. K.Tseung, 1987.
The sporophyte is a thallus, i.e. a plant without trueroots, stems or leaves. The thallus, also called the frond, iscomposed of three parts: (i) the blade or lamina, (ii) theholdfast, (iii) the stipe (Fig. 1.8).
(iv) Transplantation of young sporophytes refers to theprocedure of removing young seedlings from the seedling ropes atthe end of intermediate culture and transplanting them to thickerkelp culture ropes for final grow-out on rafts. The procedure isequivalent to transplanting rice shoots in paddy culture.
(v) Raft culture grow-out of kelp plants refers to thefinal stage of kelp production when culture ropes with transplantedsporophytes attached are suspended from floating raftropes anchored in shallow sea areas. The grow-out period innorthern China lasts eight months, from about mid-November tomid-July of the following year.
The bamboo seedling-rope method of collecting zoospores andcultivating young sporelings was very inefficient, because thebamboo rods also served as ideal substrates for zoospores ofcompeting seaweed species, such as Ectocarpus, Enteromorpha andLiemorpha. As soon as the seedling ropes were lowered into theseawater, zoospores from these other species attached and grewprolifically, covering the seedling ropes. Under naturalconditions Laminaria zoospores need about 20 days to develop intomulticelled seedlings and, because other seaweeds blocked outlight, growth of Laminaria seedlings was stunted for the firsttwo months while awaiting other species to complete their lifecycle and wither. The delay in growth of Laminaria seedlingsmeant that they were not ready for transplanting to thicker kelpropes for final raft culture until late January or earlyFebruary, during the coldest season of the year. In mid-winterin northern China the frigid seawater temperature and harshweather conditions make transplantation work very difficult.
Since 1956 six basic methods (with variations) have beendeveloped for applying fertilizers: (i) using unglazed porousclay bottles, (ii) using porous plastic bags, (iii) splashingliquid fertilizer, (iv) spraying liquid fertilizer, (v) soakingyoung sporelings in a fertilizer solution, and (vi) naturalfertilization through polyculture.
Simple splashing and sprinkling techniques were lateradvanced by using so-called spraying methods, where special boatsequipped with tanks, pumps and hoses equipped with high pressurespray nozzles are used to fertilize hundreds of acres of growingarea per day. Initial investment in equipment is high, but isdistributed between many seafarming operations since the sprayingboats rotate from farm to farm. Therefore this spraying methodof fertilizer application is able to cover large areas withfertilizer solution very efficiently and at overall lower costper acre.
Two different spore-forming methods are used in land plants, resulting in the separation of sexes at different points in the lifecycle. Seedless, non- vascular plants produce only one kind of spore and are called homosporous. The gametophyte phase (1n) is dominant in these plants. After germinating from a spore, the resulting gametophyte produces both male and female gametangia, usually on the same individual. In contrast, heterosporous plants produce two morphologically different types of spores. The male spores are called microspores, because of their smaller size, and develop into the male gametophyte; the comparatively larger megaspores develop into the female gametophyte. Heterospory is observed in a few seedless vascular plants and in all seed plants.
When the haploid spore germinates in a hospitable environment, it generates a multicellular gametophyte by mitosis. The gametophyte supports the zygote formed from the fusion of gametes and the resulting young sporophyte (vegetative form). The cycle then begins anew.
The sporophyte consists of a root and a shoot with upright leaves called fronds. Some ferns have a rhizome, a horizontal stem from which the true roots extend. Most ferns are much taller than either liverworts or mosses because fern plants are sporophtyes, which have vascular tissue; mosses and liverworts are gemetophytes, which lack vascular tissue. Vascular tissue consists of two kinds of cells. Xylem vessels transport water up from the roots; phloem sieve-tube members transport nutrients made in the leaves to other parts of the plant.
Slide: Pteris rhizome x.s. Several oval vascular bundles are encircle a central pair. Large, thick walled cells in the center of each bundle are xylem tubes; they are probably stained pink. Surrounding them are small, thin walled phloem cells, also tubular; they are probably stained green. Outside the bundles are zones of brown cells called sclerenchyma, a strengthening tissue.
The mature sporophyte forms sporangia, and within them meiosis occurs to produce haploid spores (n). The sporangia are grouped together in clusters called sori (sorus, singular) on the undersides of the leaves. They look like rusty brown rosettes.
Look at the cross section of the leaf itself. The top and bottom are defined by cellular layers called upper and lower epidermis. The inside of the leaf is called mesophyll; it is divided into two parts, closely packed tall cells above that maximize light collection for photosynthesis--palisade mesophyll--and a meshwork of cells with open spaces below for gas exchange--spongy mesophyll. Openings into the air spaces are through stomata in the lower epidermis. Recall that the pores and air spaces in Marchantia were at the top, because that plant lies on the ground. 2b1af7f3a8